1. Field of the Invention
This invention relates to electronic sensors, and more particularly to a method and apparatus for rapidly determining chemical properties of a solution using a plurality of ion-sensitive field effect transistors.
2. Description of Related Art
It is known to use ion-sensitive field effect transistors (ISFETs) to detect and measure chemical properties, such as ion activity, within a solution, such as blood. Use of ISFETs for detecting and measuring chemical properties of blood can, among other things, enhance a physician's ability to provide efficient care to a patient by providing essentially continuous information about the patient's blood chemistry.
U.S. Pat. No. 4,020,830 to Johnson et al., discloses an ISFET capable of detecting and measuring concentrations of ions, such as H.sup.+, Na.sup.+, K.sup.+, or Ca.sup.++. An ISFET operates in a manner that is similar to the operation of a standard metal oxide semiconductor field effect transistor (MOSFET) device. For example, in a standard N-channel MOSFET, two spaced apart diffusion regions having an N-type doping (one doped region being referred to as the source and the other as the drain) are located in the upper surface of a substrate. The region of the substrate that separates the source and the drain is referred to as the channel. Typically, an electrical insulating material is grown over the channel. An electrically conductive material is deposited on each of the N-type diffusion regions and on the insulating material which covers the channel, to form electrical contacts to the source and drain. A gap between the two regions of deposited electrically conductive material forms a "gate". The result is a device in which current flowing from one diffusion region to the other when an electrical potential is applied between the two diffusion regions, can be controlled by varying the voltage applied to the gate contact.
In the case of an ISFET, the structure is similar, except that a membrane which is adapted to selectively interact with a particular ion is deposited over the insulating material that covers the gate region of the substrate. The entire structure is then sealed in a material that is impervious to those solutions in which the ISFET is intended to operate, leaving only a portion of the membrane exposed. The device may then be immersed in a solution. A reference electrode coupled to the ISFET is placed into the solution and an electrical potential is applied such that ions in the solution interact with the ion-selective membrane. This creates an electrochemical voltage at the gate of the device, and an electric field in the conduction channel of the substrate. The strength of the electric field is dependent upon the ion concentration and the voltage applied to the reference electrode, and varies the conduction of electrical current from the source to the drain (i.e., drain current).
ISFETs are typically coupled to an amplifier circuit. Typically, the amplifier circuit acts as an electrical buffer between the ISFET and an output device, and translates the strength of the ion-induced electric field in the channel of the ISFET into an output (either a voltage or current). FIGS. 1A, 1B, and 1C are examples of three amplifier circuits used with ISFETs. The amplifiers of FIGS. 1A and 1B are common gate circuits.
The amplifier of FIG. 1C is a common drain circuit. Variations in the operating point of the ISFET 101 (i.e., the drain current, I.sub.d, the drain-to-source voltage, V.sub.ds, and the temperature of the ISFET 101) can introduce errors when translating the ion-induced electric field at the gate of the ISFET 101 into an output that is proportional to the properties of a solution. Therefore, it is desirable to hold the operating point constant. The circuit of FIG. 1C maintains a constant I.sub.d and V.sub.ds with changes in the properties of the solution in which the ISFET 101 is immersed. In the circuit of FIG. 1A, V.sub.ds varies as a function of the characteristics of the solution. In the circuit of FIG. 1B, I.sub.D varies as a function of the characteristics of the solution. Thus, because the amplifier of FIG. 1C operates with the least variation in the operating point, it is superior for use with an ISFET 101. However, variations in temperature still cause a shift in the operating point of an ISFET 101.
Frequently, it is desirable to measure more than one chemical property at a time, or to measure the same chemical property using a number of spaced apart ISFETs in the same solution or in different solutions which are electrically connected. As is the case in many data acquisition systems, it is desirable to use a single amplifier circuit which is selectively coupled (i.e., multiplexed) to each ISFET when a plurality of ISFETs are to be used in a system. A simple means for multiplexing ISFETs is to connect each ISFET to the data acquisition system one at a time, having all but the one disconnected. When an ISFET is disconnected, no current flows through the ISFET and the ISFET can be said to be "off".
However, when an ISFET is coupled to a data acquisition system and begins conducting electrical current I.sub.d (i.e., the ISFET is "on"), the ISFET heats due to power dissipated in the ISFET (i.e., the voltage applied between the drain and the source of the ISFET V.sub.ds multiplied by the current that flows between the drain and the source of the ISFET I.sub.d). This increase in temperature shifts the operating point, and thus alters the relationship between the electrochemical voltage at the gate V.sub.g, the drain-to-source voltage V.sub.ds, and the drain current I.sub.d, making it difficult to determine the chemical properties of the solution until the temperature stabilizes. Once the temperature of the ISFET has stabilized, the output can be calibrated. However, due to the exponential response of the temperature stabilization curve, it may be several minutes before the temperature of the ISFET settles to a drift rate small enough to yield an accurate value. This means that each time a different ISFET is switched on (i.e., coupled to an amplifier circuit), that ISFET will require several minutes to "warm up". When a large number of ISFETs are being multiplexed, the time between each consecutive reading of a particular ISFET is equal to the number of ISFETs to be read multiplied by the amount of time required for each to reach a stable temperature. In many measurement applications it is unacceptable to wait for long periods for the results of the measurement.
One way to increase the rate at which multiplexed ISFETs can be read is presented in an article by Piet Bergveld, entitled "Design Considerations for an ISFET Multiplexer and Amplifier". Bergveld proposed maintaining current I.sub.d through each ISFET except for brief time periods during which another ISFET is being read. Each ISFET can be read rapidly if the temperature remains relatively stable. The timing of the Bergveld circuit is such that each ISFET is conducting current for a relatively long period with respect to the time that each ISFET is not conducting. Therefore, when an ISFET is to be read, it is at, or very close to, a stable temperature. FIG. 2 is an electrical schematic diagram of the Bergveld circuit 200.
This solution is practical for amplifiers that operate in the common gate configuration of FIGS. 1A, or 1B. However, the Bergveld circuit 200 precludes the use of the superior common drain amplifier configuration of FIG. 1C, because in a common drain configuration, the ISFET gate is in the amplifier feedback loop. Therefore, it would be necessary to isolate the gate 208a, 208b of each ISFET 201a, 201b from the gate 208a, 208b of each other ISFET 201a, 201b when the ISFET 201a, 201b is turned on (i.e., conducting current I.sub.d) so that the current through each ISFET 201a, 201b can be maintained at the proper operating value. This isolation is impossible in the common drain configuration, since the gates of all ISFETs are commoned by the conductivity of the solution. The Bergveld circuit also necessitates the mutual isolation of all ISFET source connections, which is disadvantageous for production of ISFETs.
Therefore, there remains a need for a method and apparatus for ISFET multiplexing which takes advantage of the benefits of a common drain amplifier configuration, while allowing rapid determination of the character of the solution being measured. The following method and apparatus provides such an ISFET multiplexing circuit.